ENGLISH ABSTRACT: Large volumes of high strength wastewater are generated annually by distilleries. Treatment of this wastewater is essential to increase its reuse potential. Anaerobic digestion (AD) has been widely applied as high strength wastewater treatment. Additionally, grain distillery wastewater (GDWW) is high in fats, oils and grease (FOG) which can cause problems during AD. Pre-treatment is therefore often required to make the AD process more efficient.
The overall objective of this study was to investigate the operational efficiency of upflow anaerobic sludge blanket (UASB) reactors while treating GDWW as substrate. The first aim was to investigate the pre-treatment of the GDWW specifically for the removal of FOG by evaluating the effect of pH adaption, coagulant concentration and ozone (O3) dosages on the FOG reduction efficiency. Secondly, the effect of two different pre-treatments (only coagulant and coagulant and ozone) on the subsequent UASB treatment step was investigated.
The pH of raw GDWW (pH 3.4) was adapted to three different pH values (5.0, 6.0, and 7.0) and the coagulant, aluminium chlorohydrate (ACH) (140 mg.L-1), was added. To make the process more economically viable, the lowering the coagulant concentration (to 100 mgACH.L-1) was also investigated. Optimal reductions for chemical oxygen demand (COD) (33.2% ± 4.93), total suspended solids (TSS) (91.9% ± 1.73) and FOG (84.1% ± 1.98) were, however, achieved at a higher coagulant concentration of 140 mgACH.L-1, and at a wastewater pH of 7.0. The effect of additional ozone treatment was also investigated. Maximum reductions for the ozone treatment were obtained at 100 mgO3.L-1 for COD (3.6% ± 4.08), and at 900 mgO3.L-1 for TSS (27.7% ± 5.58) and FOG (23.9% ± 1.83). The ozone treatment was most efficient for FOG reduction (in terms of mg FOG reduced per mg ozone) at 100 mgO3.L-1. An ozone dosage of 300 mgO3.L-1, was decided on based on economic feasibility, findings in literature on toxicity of ozone and the potential degradation of recalcitrant compounds at this dosage. The final pre-treatment thus included pH adaption to 7.0, coagulant dosage of 140 mgACH.L-1, and an ozone dosage of 300 mgO3.L-1.
The second part of this study involved the operation of two 2 L laboratory scale UASB reactors for 277 days. The substrate of the first reactor contained GDWW that had only undergone coagulant pre-treatment (Rcontrol), while the substrate of the second UASB reactor consisted of GDWW that had undergone coagulant and ozone pre-treatment (Rozone). Both reactors treated the pre-treated GDWW successfully at ca. 9 kgCOD.m-3d-1. COD reductions of 96% for Rcontrol and 93% for Rozone, were achieved. FOG reductions (%) showed variations throughout the study and maximum reductions of 88% and 92% were achieved for Rcontrol and Rozone, respectively. The Rozone produced more biogas, but the methane content was similar for both reactors. The additional ozone pre-treatment did not show any added benefits to the reactor performance results. UASB granule washout in Rcontrol did, however, suggest possible toxicity of unsaturated fatty acids present in the non-ozonated substrate.
The feasibility of FOG removal was shown as both reactors successfully treated pre-treated GDWW. Ozonation, after a coagulant dosage, which resulted in further reduction in the FOG content of the GDWW, is thus not essential to ensure the success of an anaerobic digestion step. Ozonation of the pre-treated GDWW could, however, be beneficial to gas production and the efficiency of a tertiary biological process.